Anti-fatigue coagents for rubber vulcanization

ABSTRACT

Novel metal salts of (poly)citraconimide and (poly)itaconimide carboxylic acids, and a vulcanizable rubber composition comprising these novel metal salts and which, upon vulcanization, exhibits improved dynamic properties, are disclosed. Also disclosed are a sulfur-vulcanization process carried out in the presence of these metal salts and the product of said vulcanization process. The novel metal salts are represented by formulas (I) and (II), wherein R 1 , R 2  and R 3  are independently selected from hydrogen, C 1  -C 18  alkyl groups, C 3  -C 18  cycloalkyl groups, C 6  -C 18  aryl groups, C 7  -C 30  aralkyl groups and C 7  -C 30  alkaryl groups and R 2  and R 3  may combine to form a ring when R 1  is hydrogen; R 4  is selected from divalent, trivalent or tetravalent linear or branched radical chosen from a C 1  -C 18  alkyl, C 2  -C 18  alkenyl, C 2  -C 18  alkynyl, C 3  -C 18  cycloalkyl, C 3  -C 18  polycycloalkyl, C 6  -C 18  aryl, C.sub. 6 -C 30  polyaryl, C 7  -C 30  aralkyl, C 7  -C 30  alkaryl, oligomers of one or more of these radicals, and which radicals may optionally contain one or more of oxygen, nitrogen, silicon, phosphorus, sulfur, sulphone, sulfoxy and boron; B and B 1  are independently selected from the following hetero atoms: oxygen and sulfur, X is a metal selected from Mg, Ti, Zn, Cd, Sr, Ba, Fe, V, Sn, Te, Mo, Mn, Pb and Al, m is an integer from 1-3 and n is an integer of from 2-4.

This invention relates to novel metal salts of (poly)citraconimide and(poly)itaconimide carboxylic acids, and a vulcanizable rubbercomposition comprising these novel metal salts and which, uponvulcanization, exhibits improved dynamic properties. More particularly,the invention also relates to a sulfur-vulcanized rubber compositionwhich is vulcanized in the presence of particular anti-fatigue coagents,as well as to a sulfur-vulcanization process carried out in the presenceof said coagents and the use of these coagents in thesulfur-vulcanization of rubber.

In the tire and belt industries, among others, better mechanical anddynamic properties are being demanded. It has long been known that thephysiomechanical and mechanical-dynamic properties of rubber can beimproved by using a large amount of sulfur as a cross-linking agent.However, under service conditions or prolonged vulcanization, excesssulfur produces reversion which results in the shortening of crosslinksand a marked decrease in heat resistance and resistance to flexcracking, among other properties in the final product.

One of these problems, the resistance to flex cracking, also known asfatigue resistance, is solved by the addition of a coagent in accordancewith the present invention.

In order to eliminate the foregoing disadvantage, it has been proposedto add saturated carboxylic acids and their metal salts tosulfur-vulcanization systems. One example of a publication relating tothis subject is U.S. Pat. No. 4,191,671. In comparative Example 8 andExamples 11-23 improvements in fatigue resistance are shown which aresaid to result from the addition of stearic acid and salts of stearicacid with zinc, calcium, magnesium, aluminum, sodium and cobalt, to asulfur-vulcanization system. This patent also mentions the addition ofunsaturated carboxylic acid salts to a combination peroxide/sulfurvulcanization system in order to improve the abrasion resistance of therubber.

European patent application 0 191 931 suggests that the use of abismaleimide compound in combination with a sulfenamide and adithiophosphoric acid leads to further improvements in the mechanicaland anti-reversion properties of sulfur-vulcanized rubbers. The patentspecification claims that these rubbers exhibit improved resistance toreversion, resistance to heat ageing and resistance to flex cracking.However, this system is limited to vulcanization carried out in thepresence of a sulfenamide accelerator in combination with adithiophosphoric acid accelerator and is thus of limited utility inactual practice.

In the article, "Change in the Structure and Properties of VulcanizatesBased on Natural Rubber Under Prolonged Vulcanization in the Presence ofVulcanizing Systems Containing Sulfur and Bismaleimides," Chavchich, T.A., et al., Kauchuk i Rezina, vol. 4, pp. 20-3, 1981, there is disclosedthat vulcanization of natural rubber tread stocks with sulfur in thepresence of m-phenylenebismaleimide at 143° C. over a 600-minute periodgave vulcanizates with enhanced physical properties.

However, despite the fact that some of the above patents claim to reducefatigue by addition of coagents, in actual practice, these systems fallshort of the desired properties. For example, although stearic acid iswidely used in the rubber industry, there remains a need for furtherimprovements in the resistance to flex cracking for rubber articleswhich are subject to fatigue.

Accordingly, the present invention provides novel compounds which, whenemployed in sulfur-vulcanization of rubber, lead to a significant,unexpected improvement in the fatigue properties of the vulcanizedrubber composition. The novel compounds of the present invention arerepresented by the formulas I and II: ##STR1## wherein R₁, R₂ and R₃ areindependently selected from hydrogen, C₁ -C₁₈ alkyl groups, C₃ -C₁₈cycloalkyl groups, C₆ -C₁₈ aryl groups, C₇ -C₃₀ aralkyl groups and C₇-C₃₀ alkaryl groups and R₂ and R₃ may combine to form a ring when R₁ ishydrogen; R₄ is selected from divalent, trivalent or tetravalent linearor branched radical chosen from a C₁ -C₁₈ alkyl, C₂ -C₁₈ alkenyl, C₂-C₁₈ alkynyl, C₃ -C₁₈ cycloalkyl, C₃ -C₁₈ polycycloalkyl, C₆ -C₁₈ aryl,C₆ -C₃₀ polyaryl, C₇ -C₃₀ aralkyl, C₇ -C₃₀ alkaryl, oligomers of one ormore of these radicals, and which radicals may optionally contain one ormore of oxygen, nitrogen, silicon, phosphorus, sulfur, sulphone, sulfoxyand boron; B and B¹ are independently selected from the following heteroatoms: oxygen and sulfur, X is a metal selected from Mg, Ti, Zn, Cd, Sr,Ba, Fe, V, Sn, Te, Mo, Mn, Pb and Al, m is an integer from 1-3 and n isan integer of from 2-4.

The present invention also encompasses sulfur-vulcanization processescarried out in the presence of at least one compound of the formulas Iand II, sulfur-vulcanized rubbers compositions made by such processesand the use of compounds of the formulas I and II as anti-fatiguecoagents in the sulfur-vulcanization of rubbers.

The use of metal salts of, for example, methacrylic, maleic andbetaphenyl acrylic acids in the vulcanization of rubber is known fromEuropean patent application 0 390 012. In this application, the zincsalts of methacrylic acid are preferred. Further, a combinationsulfur/peroxide vulcanization system must be employed in order toachieve the object of this disclosure, namely products comprising bothionic and covalent crosslinks. This patent application does not mentionthe fatigue properties of the rubbers.

The use of zinc methacrylate and zinc salts of acrylic acid and cinnamicacid to reduce the Mooney viscosity in the compounded state of rubbercompositions is also known from U.S. Pat. No. 4,192,790. These coagentsare said to be useful in both sulfur and peroxide curing systems.

Further, it has been suggested to add a wide variety of metal salts ofunsaturated carboxylic acids to elastomeric compositions used in themaking of golf balls using a peroxide-based curing system. For example,U.S. Pat. Nos. 4,056,269; 4,065,537 and 4,264,075 suggest the use ofsalts of zinc, magnesium, calsium, lithium, sodium, potassium, cadmium,lead, barium, zirconium, berylium, copper, aluminum, tin, iron, antimonyand bismuth with unsaturated carboxylic acids. Among the wide variety ofunsaturated carboxylic acids mentioned are itaconic acid, maleic acid,substituted maleic acids, N-substituted maleamic acids, fumaric acid,crotonic acid and cinnamic acids. Also mentioned is the potential use ofmetal salts of maleimides and methylmaleimides. These compounds are saidto improve several properties of the golf balls including durability,cannon life, sound and distance properties. However, few of thesecompounds are actually exemplified in these patents.

Finally, in non-prepublished International patent applicationpublication number WO 92/07904, the use of biscitraconimides asanti-reversion coagents in the sulfur-vulcanization of rubber isdisclosed. However, this application does not teach or suggest the useof metal salts of these materials and does not address the problem ofresistance to flex cracking.

The present invention provides an excellent anti-fatigue effect withouthaving a significant adverse effect on the remaining properties of therubbers, when compared with similar sulfur-vulcanization systems usingother coagents.

The present invention is applicable to all natural and syntheticrubbers. Examples of such rubbers include, but are not limited to,natural rubber, styrene-butadiene rubber, butadiene rubber, isoprenerubber, acrylonitrile-butadiene rubber, chioroprene rubber,isoprene-isobutylene rubber, brominated isoprene-isobutylene rubber,chlorinated isoprene-isobutylene rubber, ethylene-propylene-dieneterpolymers, as well as combinations of two or more of these rubbers andcombinations of one or more of these rubbers with other rubbers and/orthermoplastics.

Examples of sulfur which may be used in the present invention includevarious types of sulfur such as powdered sulfur, precipitated sulfur andinsoluble sulfur. Also, sulfur donors may be used in place of, or inaddition to sulfur in order to provide the required level of sulfurduring the vulcanization process. Examples of such sulfur donorsinclude, but are not limited to, tetramethylthiuram disulfide,tetraethylthiuram disulfide, tetrabutylthiuram disulfide,dipentamethylene thiuram hexasulfide, dipentamethylene thiuramtetrasulfide, dithiodimorpholine and mixtures thereof.

In this text, references to sulfur shall include sulfur donors andmixtures of sulfur and sulfur donors. Further, references to thequantity of sulfur employed in the vulcanization, when applied to sulfurdonors, refer to a quantity of sulfur donor which is required to providethe equivalent amount of sulfur that is specified.

Anti-fatigue coagents of the present invention are represented by thegeneral formulas I & II. These coagents may be made by reacting apolycitraconic or polyitaconic imide acid with the oxide of the metalwhich is to be employed. In general, sufficient metal oxide is used toneutralize all of the polycitraconic or polyitaconic imide acid.

The imides of the present invention may be prepared by the methodsdisclosed in, "The synthesis of Biscitraconimides andPolybiscitraconimides," Galanti, A. V. and Scola, D. A., Journ. of Poly.Sci.: Polymer Chemistry Edition, Vol. 19, pp. 451-475, (1981); and "TheSynthesis of Bisitaconamic Acids and Isomeric Bisimide Monomers,"Galanti, A. V. et al., Journ. Poly. Sci.: Polymer Chemistry Edition,Vol. 20, pp. 233-239 (1982), by the use of (poly)amino carboxylic acidsin place of the amine starting materials.

The resulting imido carboxylic acids are converted into the salts of theinvention by addition of approximately equal equivalents of imidocarboxylic acid and metal acetate to xylene and removal of acetic acidunder reflux conditions using, for example, a Dean-Stark apparatus.

The preferred polycitraconic imide acid salts of the present inventionrepresented by the formulas I and II include, the salts wherein R₁ =R₂=R₃ =H. In a more preferred embodiment, B=B₁ =oxygen. In the mostpreferred embodiments, R₄ is an alkyl, aryl or aralkyl group, m is 1, nis 2 and X is zinc or magnesium. The same preferences apply to thepolyitaconic imide acid salts.

More specifically, the group R₄ mentioned in the formulas I and II is adivalent, trivalent or tetravalent linear or branched radical chosenfrom a C₁ -C₁₈ alkyl, C₂ -C₁₈ alkenyl, C₂ -C₁₈ alkynyl, C₃ -C₁₈cycloalkyl, C₃ -C₁₈ polycycloalkyl, C₆ -C₁₈ aryl, C₆ -C₃₀ polyary, C₇-C₃₀ aralkyl, C₇ -C₃₀ alkaryl, oligomers of one or more of theseradicals, and which radicals may optionally contain one or more ofoxygen, nitrogen, silicon, phosphorus, sulfur, sulphone, sulfoxy andboron.

More specific examples of some of the imide compounds useful in thepresent invention include, but are not limited to, the following:

Zinc Bis-(N-carboxymethyl-citraconimide)

Zinc Bis-(N-2-carboxyethyl-citraconimide)

Zinc Bis-(N-3-carboxypropyl-citraconimide)

Zinc Bis-(N-4-carboxybutyl-citraconimide)

Zinc Bis-(N-5-carboxypentyl-citraconimide)

Zinc Bis-(N-6-carboxyhexyl-citraconimide)

Zinc Bis-(N-7-carboxyheptyl-citraconimide)

Zinc Bis-(N-8-carboxyoctyl-citraconimide)

Zinc Bis-(N-9-carboxynonyl-citraconimide)

Zinc Bis-(N-10-carboxydecyl-citraconimide)

Zinc Bis-(N-11-carboxyundecyl-citraconimide)

Zinc Bis-(N-12-carboxydodecyl-citraconimide)

Zinc Bis-(N-4-carboxyphenyl-citraconimide)

Zinc Bis-(N-3-carboxyphenyl-citraconimide)

Zinc Bis-(N-2-carboxyphenyl-citraconimide)

Zinc Bis-(N-(1-carboxy-2-methyl)propyl-citraconimide)

Zinc Bis-(N-(4-carboxyphenyl)methyl-citraconimide)

Zinc Bis-(N-α-acetoxyphenyl-citraconimide)

Zinc Bis-(N-(4-glyoxy-2-thiazolyl)-citraconimide)

Zinc Bis-(N-(4-carboxy-3-pyrazolyl)-citraconimide)

Zinc Bis-(N-(3-carboxy-4-nitro)phenyl-citraconimide)

Zinc Bis-(N-(1-carboxy-3-hydroxy)phenyl-citraconimide)

Zinc Bis-(N-(3-carboxy-2-pyridinyl)-citraconimide)

Zinc Bis-(N-(1-carboxy-1-tertiary-butyl)methyl-citraconimide)

Zinc Bis-(N-(1-carboxy-2,2-dimethyl)propyl-citraconimide)

Zinc Bis-(N-tertiary-leucinyl-citraconimide)

Zinc Bis-(N-(2-carboxy-4-hydroxy)phenyl-citraconimide)

Zinc Bis-(N-(2-carboxy-2-propenyl)-citraconimide)

Zinc Bis-(N-(1-carboxy-4-hydroxy)phenyl-citraconimide)

Zinc Bis-(N-1-carboxypropyl-citraconimide)

Zinc Bis-(N-1-carboxybutyl-citraconimide)

Zinc Bis-(N-1-carboxypentyl-citraconimide)

Zinc Bis-(N-1-carboxyethyl-citraconimide)

Zinc Bis-(N-(2-carboxy-4-chloro)phenyl-citraconimide)

Zinc Bis-(N-(2-carboxy-4-bromo)phenyl-citraconimide)

Zinc Bis-(N-(2-carboxy-fluoro)phenyl-citraconimide)

Zinc Bis-(N-(2-carboxy-4,6-dichloro)phenyl-citraconimide)

Zinc Bis-(N-(3-carboxy-phenyl-1,5-diyl)-biscitraconimide)

Zinc Bis-(N-(2-carboxy-1-(4-hydroxyphenyl)propyl)-citraconimide)

Zinc Bis-(N-(2-carboxy-2-propyl)-citraconimide)

Zinc Bis-(N-oxamoyl-citraconimide)

Zinc Bis-(N-(1-carboxy-4-naphthyl)-citraconimide)

Zinc Bis-(N-(1-carboxy-2-methyl)butyl-citraconimide)

Zinc Bis-(N-(1-carboxy-3-methyl)butyl-citraconimide)

Zinc Bis-(N-(1-carboxy-4-thia)pentyl-citraconimide)

Zinc Bis-(N-(1-carboxypentyl-1,5-diyl)biscitraconimide)

Zinc Bis-(N-1-carboxy-2-methyl)propyl-citraconimide)

Zinc Bis-(N-(4-acetoxy-2-thiazolyl)-citraconimide)

Zinc Bis-(N-1-carboxyheptyl-citraconimide)

Zinc Bis-(N-1-carboxyhexyl-citraconimide)

Zinc Bis-(N-(1-carboxy-1,4-butyl)-biscitraconimide)

In the foregoing list of examples, zinc can of course be substituted byany other metal selected from Mg, Ti, Zn, Cd, Sr, Ba, Fe, V, Sn, Te, Mo,Mn, Pb and Al, and the valence of the metal will determine if the saltis a bis-, tris- or tetra-salt. Further, in all cases, the citraconimidecan also be replaced by an itaconimide to obtain the itaconimide saltsof the present invention.

The amount of sulfur to be compounded with the rubber is, based on 100parts of rubber, usually 0.1 to 25 parts by weight, and more preferably0.2 to 8 parts by weight. The amount of sulfur donor to be compoundedwith the rubber is an amount sufficient to provide an equivalent amountof sulfur which is the same as if sulfur itself were used.

The amount of anti-fatigue coagent to be compounded with the rubber is,based on 100 parts of rubber, 0.1 to 5 parts by weight, and morepreferably 0.2 to 3.0 parts by weight. These ingredients may be employedas a pre-mix, or added simultaneously or separately, and they may beadded together with other rubber compounding ingredients as well.

In most circumstances it is also desirable to have a vulcanizationaccelerator in the rubber compound. Conventional, known vulcanizationaccelerators may be employed. The preferred vulcanization acceleratorsinclude mercaptobenzothiazole, 2,2'-mercaptobenzothiazole disulfide,sulfenamide accelerators including N-cyclohexyl-2-benzothiazolesulfenamide, N-tertiary-butyl -2-benzothiazole sulfenamide,N,N'-dicyclohexyl-2-benzothiazole sulfenamide, and2-(morpholinothio)benzothiazole; thiophosphoric acid derivativeaccelerators, thiurams, dithiocarbamates, diphenyl guanidine,diorthotolyl guanidine, dithiocarbamylsulfenamides, xanthates, triazineaccelerators and mixtures thereof.

When the vulcanization accelerator is employed, quantities of from 0.1to 8 parts by weight, based on 100 parts by weight of rubbercomposition, are used. More preferably, the vulcanization acceleratorcomprises 0.3 to 4.0 parts by weight, based on 100 parts by weight ofrubber.

Other conventional rubber additives may also be employed in their usualamounts. For example, reinforcing agents such as carbon black, silica,clay, whiting and other mineral fillers, as well as mixtures of fillers,may be included in the rubber composition. Other additives such asprocess oils, tackifiers, waxes, antioxidants, antiozonants, pigments,resins, plasticizers, process aids, factice, compounding agents andactivators such as stearic acid and zinc oxide may be included inconventional, known amounts. For a more complete listing of rubberadditives which may be used in combination with the present inventionsee, W. Holmann, "Rubber Technology Handbook," Chapter 4, RubberChemicals and Additives, pp. 217-353, Hanser Publishers, Munich 1989.

Further, scorch retarders such as phthalic anhydride, pyromelliticanhydride, benzene hexacarboxylic trianhydride, 4-methylphthalicanhydride, trimellitic anhydride, 4-chlorophthalic anhydride,N-cyclohexyl-thiophthalimide, salicylic acid, benzoic acid, maleicanhydride and N-nitrosodiphenyl amine may also be included in the rubbercomposition in conventional, known amounts. Finally, in specificapplications it may also be desirable to include steel-cord adhesionpromoters such as cobalt salts and dithiosulfates in conventional, knownquantities.

The present invention also relates to a vulcanization process whichcomprises the step of vulcanizing at least one natural or syntheticrubber in the presence of 0.1 to 25 parts by weight of sulfur or asulfur donor per 100 parts by weight of rubber, characterized in thatsaid process is carried out in the presence of an effective amount of ananti-fatigue coagent represented by the formulas I and II.

The process is carried out at a temperature of 110°-220° C. over aperiod of up to 24 hours. More preferably, the process is carried out ata temperature of 120°-190° C. over a period of up to 8 hours in thepresence of 0.1 to 5.0 parts by weight of anti-fatigue coagent. Evenmore preferable is the use of 0.2-3.0 parts by weight of anti-fatiguecoagent. All of the additives mentioned above with respect to the rubbercomposition may also be present during the vulcanization process of theinvention.

In a more preferred embodiment of the vulcanization process, thevulcanization is carried out at a temperature of 120°-190° C. over aperiod of up to 8 hours and in the presence of 0.1 to 8.0 parts byweight, based on 100 parts by weight of rubber, of at least onevulcanization accelerator.

The present invention also comprises the use of a compound of theformulas I and II as an anti-fatigue coagent in the sulfur-vulcanizationof rubber. Finally, the present invention also includes articles ofmanufacture, such as tires, which comprise sulfur-vulcanized rubberwhich is vulcanized in the presence of the anti-fatigue coagents of thepresent invention.

The invention is further illustrated by the following examples which arenot to be construed as limiting the invention in any way. The scope ofthe invention is to be determined from the claims appended hereto.

EXPERIMENTAL METHODS USED IN THE EXAMPLES Compounding, Vulcanization andCharacterization of Compounds

In the following examples, rubber compounding, vulcanization and testingwas carried out according to standard methods except as otherwisestated:

Base compounds were mixed in a Farrel Bridge BR 1.6 liter Banbury typeinternal mixer (preheating at 50° C., rotor speed 77 rpm, mixing time 6min with full cooling).

Vulcanization ingredients and coagents were added to the compounds on aSchwabenthan Polymix 150L two-roll mill (friction 1:1.22, temperature70° C., 3 min).

Cure characteristics were determined using a Goettfert elastograph orMonsanto rheometer ODR (arc 1°) or MDR 2000E (arc 0.5°): delta torque orextent of crosslinking (R_(oo)) is the maximum torque (MH, also denotedas initial torque maximum, T_(i)) minus the minimum torque (ML). Scorchsafety (t_(s) 2) is the time to 2% of delta torque above minimum torque(ML), optimum cure time (t₉₀) is the time to 90% of delta torque aboveminimum, reversion time (t_(r) 2) is the time to 2% of delta torquebelow maximum torque. Final torque (T_(f)) is the torque measured afterthe overcure time.

Sheets and test specimens were vulcanized by compression molding in aFontyne TP-400 press.

Fatigue to failure was determined using a Monsanto FTFT tester (cam 14;ASTM D 4482 ).

EXAMPLES 1-2 and COMPARATIVE EXAMPLES A-D

Two different anti-fatigue agents in accordance with the presentinvention were prepared and tested in the sulfur vulcanization processaccording to the present invention. The citraconic acid salts employedare listed in Table 1. These coagents were compared with a system withno coagent (control), the zinc salt of a monocitraconimide (MCI-CPHZ),Duralink® HTS and a meta-xylylene biscitraconimide coagent (BCI-MX).

The formulations were cured at 150° C. or 170° C. until t₉₀ was reached.

The accelerator employed was n-cyclohexyl-2-benzothiazole sulfenamide(CBS). Comparative example B was a control example with no anti-fatigueadditive. Natural rubber was vulcanized in the presence of the foregoingcompounds using the formulations listed in Table 1.

The results of fatigue to failure data are given in Tables 2 and 3.Details on other physical properties are given in Table 4.

                  TABLE 1                                                         ______________________________________                                        Compound Composition                                                                     Recipes                                                                       A      B      C        1    2                                      ______________________________________                                        Ingredients                                                                   NR SMR CV    100      100    100    100  100                                  Carbon black 50       50     50     50   50                                   N-330                                                                         Stearic Acid 2        2      2      2    2                                    Zinc Oxide   5        5      5      5    5                                    Aromatic Oil 3        3      3      3    3                                    (Ingralen ® 150)                                                                       0.6      0.6    0.6    0.6  0.6                                  Perkacit ® CBS                                                            Sulfur       2.3      2.3    2.3    2.3  2.3                                  Duralink ® HTS                                                                         --       1.0    --     --   --                                   BCI-MX       --       --     1.0    --   --                                   BCI-CMZ      --       --     --     1.0  --                                   BCI-CPhz     --       --     --          1.0                                  Structures of the BCI-Zn salts                                                 ##STR2##                                                                     BCI-CMZ                                                                        ##STR3##                                                                     BCI-CPhZ                                                                      ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Failure Properties of The Vulcanizates cured at 150° C. for t90        Monsanto Fatigue to Failure Data                                              Recipes       No of Kilocycles to failure                                     ______________________________________                                        A (CONTROL)   25.8                                                            B (HTS)       27.6                                                            C (BCI-MX)    23.6                                                            1 (BCI-CMZ)   48.8                                                            2 (BCI-CPhZ)  53.9                                                            ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Failure Properties of The Vulcanizates cured at 170° C. for t90        Monsanto Fatigue to Failure Data                                              Recipes       No of Kilocycles to failure                                     ______________________________________                                        A (CONTROL)   24.6                                                            B (HTS)       27.2                                                            C (BCI-MX)    25.2                                                            1 (BCI-CMZ)   42.6                                                            2 (BCI-CPhZ)  44.2                                                            ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                               Modulus (MPa)                                                                              Tensile     Elonga-                                              50%  100%    300%    Strength (MPa)                                                                          tion(%)                                 ______________________________________                                        A        1.74   3.74    17.92 24.40     406                                   (Control)                                                                     B        1.69   3.39    16.94 25.21     428                                   (HTS)                                                                         C        1.64   3.34    16.50 24.91     433                                   (BCI-MX)                                                                      1        1.74   3.60    17.09 25.32     480                                   (BCI-CMZ)                                                                     2        1.78   3.51    16.68 25.10     465                                   (BCI-CPhZ)                                                                    ______________________________________                                    

EXAMPLES 3-4

The procedure of Examples 1-2 was repeated using the formulations givenin Table 5. The properties of the cured rubber were measured and can befound in Table 6.

                  TABLE 5                                                         ______________________________________                                        Ingredients   Control      3      4                                           ______________________________________                                        NR SMR CV     100          100    100                                         Carbon Black  50           50     50                                          N-330                                                                         Stearic Acid  3            3      3                                           Zinc Oxide    5            5      5                                           Aromatic Oil  3            3      3                                           (Ingralen 150)                                                                Perkacit ® CBS                                                                          0.6          0.6    0.6                                         Sulfur        2.3          2.3    2.3                                         BCI-CMZ       --           2.0    --                                          BCI-CMMg      --           --     2.0                                         ______________________________________                                    

BCI-CMMg is the same as BCI-CMZ except that the zinc ion is replaced bya magnesium ion.

                  TABLE 6                                                         ______________________________________                                        Extent of                    Kilocycles to failure                                   Cosslinking                                                                             ts2     t90            cured at                              Example                                                                              (Nm)      (min)   (min) cured at 150°                                                                   170°C.                         ______________________________________                                        Control                                                                              1.59      4.79    13.6  24.9     22.9                                         (1.42)    (1.11)  (3.1)                                                3      1.68      5.58    18.3  46.8     43.0                                         (1.47)    (1.21)  (4.3)                                                4      1.70      5.55    16.7  32.7     30.7                                         (1.40)    (1.23)  (4.3)                                                ______________________________________                                    

Values in parenthesis are for curing at 170° C. Other values are forcuring at 150° C.

These examples demonstrate the anti-fatigue properties of the zinc andmagnesium salts of the invention.

The foregoing examples were presented for the purpose of illustrationand description only and are not to be construed as limiting the scopeof the invention in any way. The scope of the invention is to bedetermined from the claims appended hereto.

What is claimed is:
 1. A sulfur-vulcanized rubber composition whichcomprises the vulcanization reaction product of a compositioncontaining:A) 100 parts by weight of at least one natural or syntheticrubber; B) 0.1 to 25 parts by weight of sulfur and/or a sufficientamount of a sulfur donor to provide the equivalent of 0.1 to 25 parts byweight of sulfur; and C) 0.1 to 5.0 parts by weight of a coagentrepresented by the formulas I and II: ##STR4## wherein R₁, R₂ and R₃ areindependently selected from hydrogen, C₁ -C₁₈ alkyl groups, C₃ -C₁₈cycloalkyl groups, C₆ -C₁₈ aryl groups, C₇ -C₃₀ aralkyl groups and C₇-C₃₀ alkaryl groups and R₂ and R₃ may combine to form a ring when R₁ ishydrogen; R₄ is selected from divalent, trivalent or tetravalent linearor branched radical chosen from a C₁ -C₁₈ alkyl, C₂ -C₁₈ alkenyl, C₂-C₁₈ alkynyl, C₃ -C₁₈ cycloalkyl, C₃ -C₁₈ polycycloalkyl, C₆ -C₁₈ aryl,C₆ -C₃₀ polyaryl, C₇ -C₃₀ aralkyl, C₇ -C₃₀ alkaryl, oligomers of one ormore of these radicals, and which radicals may optionally contain one ormore of oxygen, nitrogen, silicon, phosphorus, sulfur, sulphone, sulfoxyand boron; B and B¹ are independently selected from the following heteroatoms: oxygen and sulfur, X is a metal selected from Mg, Ti, Zn, Cd, Sr,Ba, Fe, V, Sn, Te, Mo, Mn, Pb and Al, m is an integer from 1-3 and n i san integer of from 2-4.
 2. The sulfur-vulcanized rubber composition ofclaim 1 wherein said rubber composition further comprises 0.1 to 8.0parts by weight of a vulcanization accelerator.
 3. The sulfur-vulcanizedrubber composition of claim 1 wherein R₁, R₂ and R₃ of the coagent arehydrogen, and B and B¹ oxygen.
 4. A process for the vulcanization, at atemperature of from 110° to 220° C. for up to 24 hours, of avulcanizable composition comprising at least one natural or syntheticrubber in the presence of 0.1 to 25 parts by weight of sulfur or asufficient amount of a sulfur donor to provide the equivalent of 0.1 to25 parts by weight of sulfur, characterized in that said process iscarried out in the presence of an effective amount of an anti-fatiguecoagent represented by the formulas I and II: ##STR5## wherein R₁, R₂and R₃ are independently selected from hydrogen, C₁ -C₁₈ alkyl groups,C₃ -C₁₈ cycloalkyl groups, C₆ -C₁₈ aryl groups, C₇ -C₃₀ aralkyl groupsand C₇ -C₃₀ alkaryl groups and R₂ and R₃ may combine to form a ring whenR₁ is hydrogen; R₄ is selected from divalent, trivalent or tetravalentlinear or branched radical chosen from a C₁ -C₁₈ alkyl, C₂ -C₁₈ alkenyl,C₂ -C₁₈ alkynyl, C₃ -C₁₈ cycloalkyl, C₃ -C₁₈ polycycloalkyl, C₆ -C₁₈aryl, C₆ -C₃₀ polyaryl, C₇ -C₃₀ aralkyl, C₇ -C₃₀ alkaryl, oligomers ofone or more of these radicals, and which radicals may optionally containone or more of oxygen, nitrogen, silicon, phosphorus, sulfur, sulphone,sulfoxy and boron; B and B¹ are independently selected from thefollowing hetero atoms: oxygen and sulfur, X is a metal selected fromMg, Ti, Zn, Cd, Sr, Ba, Fe, V, Sn, Te, Mo, Mn, Pb and Al, m is aninteger from 1-3 and n is an integer of from 2-4.
 5. The vulcanizationprocess of claim 4, wherein said rubber is vulcanized in the furtherpresence of 0.1 to 8.0 parts by weight of a vulcanization accelerator.6. The vulcanization process of claim 4 wherein R₁, R₂ and R₃ of saidcoagent are hydrogen and B and B₁ are oxygen.
 7. An article ofmanufacture comprising a sulfur-vulcanized rubber product made by theprocess of claim
 4. 8. The sulfur-vulcanized composition of claim 1wherein R₄ of said coagent is selected from the group consisting ofmethylene, ethylene, pentamethylene, isopropylmethylene and phenylene.9. The sulfur-vulcanized composition of claim 1 wherein n of saidcoagent is 2 and X is selected from the group consisting of Zn and Mg.10. The vulcanization process of claim 4 wherein R₄ of said coagent isselected from the group consisting of methylene, ethylene,pentamethylene, isopropylmethylene and phenylene.
 11. The vulcanizationprocess of claim 4 wherein n of said coagent is 2 and X is selected fromthe group consisting of Zn and Mg.